Mechanical scoring and separation of strengthened glass

ABSTRACT

A strengthened glass sheet is separated into undamaged sheet segments by mechanically scribing one or more vent lines of controlled depth into the sheet surface, the depths of the scribed lines being insufficient to effect sheet separation, and then applying a uniform bending moment across the vent lines to effect separation into multiple sheet segments, the vent lines being scribed from crack initiation sites comprising surface indentations formed proximate to the edges of the glass sheet.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority under 35 U.S.C. §119(e)of U.S. Provisional Application Ser. No. 61/315,491 filed on Mar. 19,2010.

BACKGROUND

1. Field

The present disclosure relates generally to methods for cutting andseparating strengthened glass sheets into smaller sheet segments, andmore specifically to methods for mechanical cutting and separatingstrengthened glass substrates without inducing undesired sheet breakage.

2. Technical Background

Chemically strengthened glass sheet has been used in a wide range ofapplications including protective cover glasses for consumer electronicdevices. The ion-exchange process used for chemical strengtheningcreates a layer of compressive stress on glass surfaces that providesthe desired increase in surface damage resistance, but at the same timeresults in a tensile stress in the mid-section across the thickness ofglass.

To obtain chemically strengthened glass sheet components in accordancewith current practice, the components are first cut as sheet segmentsfrom non-strengthened (non-ion exchanged) glass sheet into the finalshape for the desired component, with finishing of the segment edges andshapes to meet aesthetic and functional objectives. Thereafter the glasscomponents go through the ion-exchange strengthening process byimmersing the sheet segments into an ion-exchange bath at an optimumelevated temperature and for a time sufficient to develop an engineeredstress profile across glass thickness that provides the required surfacestrengthening effect. Thereafter the components are removed from thebath and cleaned for further processing.

As the applications for chemically strengthened glass widen to coveremerging technologies, such as devices with integrated touch screens,display manufacturers require the ion-exchanged glass to be supplied inlarger sheets of glass, for subsequent cutting into the various sizesand shapes of the final components. To reduce the number of componentsused to support the functionality of touch screen devices, and to lowermanufacturing costs, it is increasingly required that the cutting andseparation process steps for medium-to-large-size glass panels beconducted on panels previously subjected to ion-exchange strengthening.

Currently available mechanical methods and processes for cutting glasssheet, including thin drawn glass sheet of the kind used for small andlarge information display panels, have not yet been successfully adaptedto the cutting of ion-exchange-strengthened glass without causing glasscracking, due to the relatively high frangibility of many chemicallystrengthened glasses. Thus increased attention is presently beingfocused on specialized processes, such as water-jet cutting and theso-called “wet-etching” processes, to enable the efficient cutting andseparation of chemically strengthened glasses. Many of these proceduresare time-consuming and expensive, however, so there remains a need foran economical yet effective method for separating relatively frangiblechemically strengthened glass sheet into sheet segments of predeterminedsizes and shapes.

SUMMARY

In accordance with the present disclosure, methods for the cutting andseparation of large-sheet chemically strengthened glass into smallerstrengthened sheet segments of predetermined size and shape areprovided. Economical mechanical scoring process parameters, incombination with improved separation techniques, can secure clean sheetseparation even in thin glass sheets incorporating high levels ofcompressive surface stress. Further, the disclosed methods can readilybe adapted for use in presently existing manufacturing environments.

In a first aspect, therefore, the disclosure encompasses methods forscribing a strengthened glass sheet without inducing sheet separation orbreakage. Those methods comprise, first, forming a crack initiation sitein a first surface of the glass sheet, at a location proximate to afirst edge of the sheet. The glass sheet is chemically strengthenedsheet having a surface compression layer of a given depth (a depth“DOL”), and the crack initiation site formed at the location proximateto the first edge of the sheet comprises a surface indentation extendinginto the first surface.

Following the formation of the crack initiation site, the first surfaceis mechanically scored from the crack initiation site toward a secondedge of the glass sheet such as the opposite edge of the sheet to scribea vent line in the surface. For the purpose of the present description avent line is an indentation line formed in the sheet surface that opensthat surface to a certain depth. In accordance with the present methodsthe vent line extends into the first surface of the sheet to a ventdepth at least equal to the depth DOL, but less than a fracture depth,i.e., a depth causing a spontaneous and complete separation of the glasssheet into sheet segments or other pieces.

In a second aspect the disclosure provides methods for separatingchemically strengthened glass sheet incorporating surface compressionlayers into two or more sheet segments. Those methods utilize vent linesprovided in accordance with the foregoing description and/or by othersuitable procedures. In accordance with those methods a vent line isfirst provided on a first surface of the sheet, that vent line extendinginto the surface to a vent depth at least equal to the depth of thesurface compression layer (e.g., a depth DOL), but less than a fracturedepth causing separation of the glass sheet. Thereafter a uniformbending moment is applied across the vent line on the first surface, thebending moment being of a sufficient magnitude (i.e., generatingsufficient surface stress) to separate the glass sheet into glasssegments along the vent line. A fracture initation site or sites such asabove described may be provided at vent line ends to improve separationefficiency if desired.

BRIEF DESCRIPTION OF THE DRAWINGS

Other aspects of the presently disclosed methods are described below, inpart with reference to the appended drawings wherein:

FIG. 1 and FIG. 1 a schematically illustrate the mechanical formation ofa crack initiation site proximate to the edge of a chemicallystrengthened glass sheet;

FIG. 2 illustrates an arrangement of criss-crossing vent lines forseparating a strengthened glass sheet into multiple sheet segments;

FIG. 3 illustrates an arrangement for a glass sheet separation assemblyincluding a compliant supporting sheet for applying 3-point bendingstress to a scribed glass sheet supported thereon;

FIG. 4 is an elevational view of a section of a fracture surface of aglass sheet segment produced in accordance with the methods of thepresent disclosure, and

FIG. 5 compares the separation efficiencies of the disclosed methods asa function of selected process variables.

DETAILED DESCRIPTION

Although the methods disclosed herein are generally applicable to thesegmentation of a wide variety of sheet glass articles and materialsinto smaller sheet glass segments, particular embodiments of thosemethods as hereinafter more fully described incorporate processing stepsbounded by particular ranges of parameters that are especially effectivefor the scribing and segmenting highly strengthened and toughenedglasses such as are presently demanded for information displays to besubjected to severe conditions of use. Thus the following descriptionsand illustrative examples are particularly directed to such embodimentseven though the practice of the disclosed methods is not limitedthereto.

Chemically strengthened glass sheets are essentially glass substratesincorporating engineered stress profiles across their thicknesses, withsurface portions of the sheets being brought to high levels ofcompressive stress, and interior portions into tensile stress, in thecourse of chemical strengthening. Glass sheets having thicknesses notexceeding 1.5 mm, when comprising surface compression layers exhibitinga peak compressive stress in the range of 400-900 MPa and a depth ofcompression layer (DOL) in the range of 5-100 μm, are typical of thesheets being employed for information display applications. Thus theycomprise an important category of technical materials for which thepresently disclosed methods of sheet separation offer particulareconomic and technical advantages.

The scoring of such sheets utilizing techniques developed for theseparation of large glass sheets such as used for LCD display substratestypically leads to glass cracking. According to theory, when a scoringforce is applied to an ion-exchange-strengthened glass article that isadequate to overcome the surface compressive stress and initiate mediancracking into the tensilely stressed interior of the article, the cracksimply propagates uncontrollably to cause spontaneous breakage of thearticle.

In accordance with selected embodiments of the disclosed methods,however, an abrasive scoring wheel with an appropriately selectedgeometry is used to initiate controlled mechanical damage that canovercome the residual surface compressive stress without undesired sheetseparation or breakage. In particular embodiments a 2 mm-diameterdiamond scribing wheel with a suitably tapered circumferential scoringedge can be used for the purpose. As a particular example, a scribingwheel with an edge taper angle in the range of 90-140°, or even110-115°, can effect the successful scribing of a non-crack-propagatingvent line in a 1.1 mm thick sheet of ion-exchanged glass having amaximum (outer surface) compressive stress in the range of 400-900 MPa,particularly in cases where the compressively stressed surface layershave layer depths (DOL) in the range of 5-100 μm. These wheel taperangles can provide median crack depths (vent depths) equal to as much as10-20% of glass thickness without causing self-separation of the glasssheet during the scribing process.

The selection of any particular wheel taper angle within these rangescan be guided by the particular scoring speed and scoring wheel force(stress level against the glass surface) selected for vent line scoring.As illustrative embodiments of suitable scoring parameters, scoringspeeds in the range of 50-500 mm/s and scribing loads in the range of10-30N, e.g., a scoring speed of 150 mm/s and a scribing load in therange of 18-21 N, can be used.

In accordance with selected embodiments of the disclosed methods,however, an abrasive scoring wheel with an appropriately selectedgeometry is used to initiate controlled mechanical damage that canovercome the residual surface compressive stress without undesired sheetseparation or breakage. In particular embodiments a 2 mm-diameterdiamond scribing wheel with a suitably tapered circumferential scoringedge can be used for the purpose. As a particular example, a scribingwheel with an edge taper angle in the range of 90-140°, or even110-115°, can effect the successful scribing of a non-crack-propagatingvent line in a 1.1 mm thick sheet of ion-exchanged glass having amaximum (outer surface) compressive stress in the range of 400-900 MPa,particularly in cases where the compressively stressed surface layershave layer depths (DOL) in the range of 5-100 μm and the level ofcentral tension (CT) is less than 50 MPa. These wheel taper angles canprovide median crack depths (vent depths) equal to as much as 10-20% ofglass thickness without causing self-separation of the glass sheetduring the scribing process.

Suitably configured surface indentations providing effective crackinitiation sites can be formed using the same scoring wheels as used toextend the vent lines from those crack initiation sites to opposingedges of the glass sheets. Generally such crack initiation sites arecreated by edge-crushing or edge-scoring the sheets at scoring wheelheights set to produce surface indentations of a depth (h) that is lessthan about 10% of the glass sheet thickness in depth. Close control overthe vertical height difference between the tapered wheel edge and thesurface of the glass sheet is needed to control this indentation depth.

In particular embodiments, indentation depths (h) will fall in the rangeDOL≦h≦3DOL, where DOL is the depth of the surface compression layerprovided on the strengthened glass sheet. In that range the indentationsneed to extend only a short distance inwardly from the edges of thesheets.

Also useful in controlling the level of edge and surface damage at crackinitiation sites is the use of relatively low scribing speeds. Thus toavoid excessive crack propagation from those sites that could causespontaneous sheet cracking or sheet separation in the course scribingvent lines in the sheets, crack initiation scribing speeds willgenerally be lower in mm/sec than the scribing speed employed for ventline scoring. The size and shape of the resulting surface indentationscan vary, but indentation shapes in the form of “half-penny” or“semi-circular” flaws that are absent radial cracks extending from theindentations provide good results under most scribing and separatingconditions.

FIGS. 1 and 1 a of the drawings schematically illustrate the step ofproviding a crack initiation site proximate to the edge of a chemicallystrengthened glass sheet 10 in accordance with the disclosed methods. Asshown in FIG. 1, a surface indentation 12 of a depth (h) forming asuitable crack initiation site is cut into the edge and top surface 10 aof the sheet using a rotating abrasive scoring wheel 20. The wheelemployed for the purpose has a tapered circumferential cutting edge 22with a taper angle α as shown in an edge view of wheel 20 presented inFIG. 1 a. Wheel 20 or a similar scoring wheel can also be used to scribevent lines of predetermined depth extending from similar crackinitiation sites across the top surface of a glass sheet such as sheet20 utilizing, for example, a scoring wheel having a cutting edge taperangle α, a scoring speed, and a scribing load such as hereinabovedescribed.

The disclosed mechanical scoring methods include particular embodimentswherein a plurality of vent lines extending from a plurality of crackinitiation sites are scribed in the first surface of the glass sheet. Insome of these embodiments at least one vent line crosses at least oneother vent line in the surface of the sheet. In the case ofcriss-crossing vent lines or otherwise, one useful approach is to alignthe initial scribing direction across the longer dimension of the glasssheet, and to form the crossing vent lines across the shorter dimensionof the sheet. Thus where the glass sheet has a width W and a length Lthat is greater than the width W, the step of mechanical scoring cancomprise first scribing a plurality of spaced vent lines from aplurality of crack initiation sites along directions parallel to thelength L. Subsequent mechanical scoring then comprises cross-scribingone or more vent lines from similar crack initiation sites in adirection parallel to the sheet width W.

FIG. 2 of the drawings schematically illustrates a chemicallystrengthened glass sheet 30 wherein a plurality of vent lines 34 havebeen scribed into sheet top surface 30 a. The configuration of sheet 30is such that sheet length L is greater than sheet width W. In thescribing of criss-crossing vent lines 34 in such sheets according toparticular embodiments of the disclosed scribing methods, mechanicalscoring is advantageously first conducted in direction 1 to form ventlines parallel to length L of the sheet. The cross-scribing of theremaining vent lines is thereafter carried out by mechanical scoring indirection 2 lying parallel to width W of the sheet. A high level ofcross-cut corner and segment edge quality is secured in glass segmentsseparated from the sheet utilizing this particular cross-scribingapproach.

For the purpose of separating scribed sheets of chemically strengthenedglass into strengthened sheet segments, mechanical breaker apparatusrather than hand breaking is employed. In general, hand breaking cannotgenerate bending stresses that are sufficiently uniform along thelengths and across the widths of the scribed sheets to guide crackpropagation from one sheet edge to another without risking crackdeviation and thus uncontrolled sheet breakage.

Mechanical breaker arms can apply uniform bending moments/separationforces across scribe lines along the entire lengths of those lines.Further, close control over the magnitudes and rates of application ofthe applied bending/separation forces can be secured through such means.The levels of bending force to be employed for the controlled separationof sheets provided with vent lines as herein described will depend onvariables such as the thicknesses and elastic properties of the glass,the stress profiles present across the thicknesses of the strengthenedsheets, and the depths and widths of the vent lines. However, theparticular bending rates and forces to be selected in any particularcase can readily be determined by routine experiment.

The selected bending forces may be applied by cantilever beam bending orby three-point bending, but in all cases are applied in a directioneffective to develop tensional stress across the vent lines provided inthe glass surface. As examples of particular embodiments of sheetseparation methods such as herein described, tensional bending stressesof 10-35 MPa, or in the narrower range of 20-25 MPa, will be used. Inthree-point bending, the tensional stress is generated through theapplication of a mechanical force to a second surface of the glass sheetopposite the first surface.

In particular three-point-bending embodiments of the disclosedseparation methods, the scribed sheets are supported on a compliantsupport surface during the application of a mechanical breaking force tothe sheets. The sheets are positioned on the supports with the ventlines facing the support surfaces, and mechanical breaking forces arethen applied to the sheets on the sheet surfaces opposite the ventlines.

In breaking methods involving 3-point bending against compliant supportsurfaces, the desired flexure and breakage of the sheets along ventlines is favored by providing surface grooves in the support surfaces.The vent lines in the sheet surface in those embodiments are positionedover and in alignment with the grooves in the support surfaces tofacilitate sheet deflection during the application of the mechanicalforce.

FIG. 3 of the drawings presents a schematic exploded view of anarrangement for securing sheet separation via breakage in three-pointbending utilizing a compliant support surface such as described. Asshown in FIG. 3, a glass sheet 40 comprising a plurality ofcriss-crossing vent lines 44 on bottom surface 40 a of the sheet ispositioned over a support member 50 comprising a compliant supportsurface 50 a. Support surface 50 a incorporates an array of grooves 50 bwith a groove spacing corresponding to the spacing of a subset of ventlines 44 on the bottom surface of glass sheet 40. A breaker bar 60 ispositioned above the upper surface of glass sheet 40 and in alignmentwith both leftmost vent line 44 in bottom surface 40 a of glass sheet 40and leftmost groove 50 b in compliant support surface 50 a.

When the elements of the illustrated assembly are brought into contactwith each other, the breaker bar can effect separation of glass sheet 40along leftmost vent line 44 through the application of a downward forceF to the top surface of sheet 40. That force creates tensional stressacross leftmost vent line 44 through the flexure of sheet 40 downwardlyinto groove 50 b of compliant support member 50, resulting in sheetseparation caused by three-point bending of the sheet. In someembodiments, those portions of the cross-vented glass sheet 40 not beingsubjected to downward force by breaker bar 60 can be stabilized throughthe application of a uniform clamping pressure broadly applied to glasssheet 40 and underlying compliant support member 50 over the clampingarea indicated as area A in FIG. 3.

Variations in processing parameters and equipment specifications usefulin the practice of selected embodiments of the disclosed methods arepresented in the following illustrative descriptions. One considerationaffecting scribing effectiveness in some cases relates to the choice ofrotating abrasive scoring wheels for forming the scribed vent lines.Steeper wheel taper angles, e.g., taper angles in the range of 110-125°can be effectively employed with wheels of relatively small diameter,i.e., diameters in the 2-4 mm range. Ground abrasive wheel surfaces, asopposed to surfaces that have been finished by polishing, areparticularly suitable for the scribing of highly compressed glasssurfaces, and are commercially available. Suitable materials forfashioning abrasive scoring wheels or other scoring implements includetungsten carbide and diamond, the latter including either single crystalor polycrystalline diamond (PCD). Scoring wheel surface finishes in therange of 0.25 μm (+/−0.15 μm) as measured by Zygo white lightinterferometer provide good results.

The use of such wheels in combination with appropriately selectedscoring speeds and scribing loads help to prevent full-body sheetseparation during the scribing process, and to minimize surface chippingadjacent separated surfaces. The use of a computer controlled visionsystem to position the strengthened sheets on scribing tables is helpfulto insure proper sheet alignment and vent line registration and spacing.

As noted above, scribing loads and speeds suitable for producing ventlines with median crack depths in the range of 10-20% of glass thicknessare generally effective both to avoid sheet separation during scribingand to secure consistent sheet separation under subsequently appliedbending stresses. For the purpose of achieving good control over ventline depth it is generally found that increases in scribing speed thatare offset by increases in scribe load can produce equivalent ranges ofmedian crack depth.

Table 1 below provides one illustration of this effect. Included in thatTable are data covering a particular range of scribing speeds and loads,including an indication in each case as to the effectiveness of each setof scribing conditions for achieving controlled rather than spontaneoussheet separation. The data in Table 1 are representative of resultsgenerated during the scribing of ion-exchange strengthened glass sheetof 1.1 mm thickness having a peak compressive surface stress level inthe range of 600-750 MPa, a peak central tension level of less than 40Mpa, and a depth of surface compression layer in the range of 25-40 μm.Scribing conditions yielding stable criss-crossing vent lines areidentified by an XC indicator, while conditions yielding stable parallelvent lines are identified by S indicator. Conditions resulting in anundesirable level of sheet separation during scribing are identified byan FS indicator in the Table.

TABLE 1 Scribing Process Conditions Scribe Speed Scribe Load (N) (mm/s)14 18 21 24 28  65 XC FS 125 S XC XC FS 255 S XC FS

Of course, the scribing conditions most effective to generate vent lineswithout spontaneous sheet separation and/or excessive surface damage inany particular case will depend on glass thickness as well as on theparticular engineered stress profile developed across the thickness ofthe strengthened sheet in the course of chemical strengthening. Inparticular illustrative embodiments of the disclosed methods,ion-exchange strengthened glass sheet of 1.1 mm thickness having a peakcompressive surface stress level in the range of 400-850 MPa, a peakcentral tension level not exceeding 40 Mpa, and depth of compressivelayer in the range of 10-70 μm can be effectively scribed within ascribe load range of 14-24N and a scribing speed of 50-750 mm/s.Scribing within a scribe load range of 16-21 N at scribing speeds in therange of 125-250 mm/s can produce median crack depths in the range of120-180 μm in such glasses, while maintaining lateral crack sizes below150 μm.

A specific illustrative embodiment of the application of the disclosedsheet separation procedures is provided in FIG. 5 of the drawings. FIG.5 graphically presents a process window applicable to the scribing andseparation of 1.1 mm-thick strengthened glass sheet incorporating asurface compressive stress of 650 MPa, a central tension level of 30Mpa, and depth of compressive layer of 46 μm. The scribing loadsemployed during the processing of this sheet are reported as scoringpressures in MPa on the horizontal axis of the graph, while theresulting vent line depths are plotted in mm on the vertical graph axis.The plotted vent depths vary as a function of scribing speed (thesebeing in the range of 250-500 mm/sec for the results shown) as well ason scoring pressure.

Sheet separation effectiveness within the process ranges shown in FIG. 5falls into one of three regions in the graph, indicated by labels A, B,and C in the field of data. Region A corresponds to a processing regionof limited sheet separability, i.e., a region wherein the level ofseparation without segment breakage or separation edge damage is toohigh. Region C corresponds to a processing region where the incidence ofspontaneous sheet separation during scoring or scored sheet handling isexcessive. Thus best results in terms of scored sheet stability andclean sheet separation for strengthened glass sheet of this particularthickness and stress profile are secured by confining processingconditions to those falling within Region B of the graph.

The effective separation of strengthened glass sheet segments fromappropriately scribed glass sheets of larger dimensions depends in parton the application of bending moments to the scored sheets that generatetensional breaking stresses in a suitable stress range. In typicalembodiments of the sheet separation methods disclosed herein, tensionalstresses in the range of about 10-35 MPa are effective to provideconsistent sheet separation over a relatively wide range of sheetthicknesses and vent line depths. Thus, for ion-exchange-strengthenedsheets of 1.1 mm thickness, applied tensional stresses in the range of20-30 MPa across the vent lines are effective.

The selection of appropriate stress levels for achieving consistentsheet separation in three-point bending against a compliant supportingsurface will depend in part on the presence and configuration of surfacegrooves in the surface that are aligned with the vent lines in thestrengthened sheets. Suitable ranges of groove depth can be estimatedutilizing the following formula relating deflection δ of a glass sheetof thickness t resulting from the application of an applied bendingstress σ_(f) where the strengthened glass has an elastic modulus E:

$\delta = \frac{\sigma_{f}L^{2}}{6{Et}}$

Table 2 below presents calculated sheet deflections that would resultfrom the application of two levels of bending stress to strengthenedglass sheet of three different thicknesses. Bending or breaking stressesare reported in MPa, glass sheet thicknesses in mm, and the sheetdeflection at each given sheet thickness and bending stress in μm.Utilizing a compliant support material such as a silicone elastomer toprovide compliant supporting surface for effecting scribed sheetseparation in three-point bending, it can be calculated that a groovedepth greater than 2 mm in combination with a groove width greater than200 μm can accommodate sheet deflections greater than 1.1 mm.

TABLE 2 Glass Sheet Deflections Glass Thickness Breaking (mm) Stress(MPa) 0.55 0.7 1.1 15 60 50 30 25 110 80 50

The consistency of sheet separation under cantilever or three-pointbending can also be affected by the rate at which the bending forces areapplied to the glass. The selection of a suitable separation speed willdepend on sheet thickness and on the level of separation force to beapplied, with thicker and stiffer glass sheet benefiting from higherforce levels applied at lower speeds, and thinner, more flexible sheetbenefiting from the application of lower bending forces at higherspeeds. As an illustrative example, the separation of a chemicallystrengthened glass sheet of a thickness below 1.5 mm having acompressive surface stress in the range of 400-850 MPa and a scribedvent depth in the range of 100-225 μm can generally be effected at anapplied stress in the range of 10-35 MPa. Where the applied stress is tobe generated via an advancing breaker bar under cantilever beam loading,consistent sheet separation can be achieved at bar advance rates inexcess of 0.02 inches/min at a moment arm length or bar offset ofbetween 3-10 mm between the breaker bar contact line and the vent line.

An illustrative embodiment of strengthened glass sheet separationutilizing embodiments of the methods hereinabove described is shown inthe following non-limiting example.

Example

Two strengthened glass sheet types are selected for processing. Bothsamples comprise Corning Gorilla® Glass sheet of 1.1 mm sheet thickness.One sheet sample incorporates surface compression layers of 30 μmthickness and a surface compressive stress level of 750 MPa, with acalculated sheet center tension of 33 MPa. The other sheet incorporatessurface compression layers of 36 μm thickness and a surface compressivestress level of 625 MPa, also with a calculated sheet center tension of33 MPa.

Sheets of these two glasses, each 370 mm by 470 mm in size are selectedfor sectioning into four equally sized smaller sheet segments, eachsegment to be 135 mm×235 mm in size. For that purpose each sheet ismechanically scribed in accordance with the procedure below utilizing acommercial abrasive glass cutting machine, i.e., a Gen-3 TLCPhoenix-600® glass cutting machine commercially obtained from TLCInternational, Phoenix, Ariz., USA. The abrasive cutting wheel used forscoring the surfaces of the glass sheets is a DCW-TX 2.0×0.65×0.8×110°A140 tapered cutting wheel of taper angle 110°.

To provide crack initiation sites in each of the glass sheets, thesheets are first “edge-crushed” by offsetting the abrasive scoring wheelposition to a level 0.0035 inches or 90 μm below the level of the glasssheet surfaces. Crossing vent lines extending from the crack initiationsites and suitable for dividing each sheet into four smaller segmentsare then scribed into the surfaces of each of the sheets. Scribing iscarried out under a scribing pressure in the range of about 0.11-0.13MPa, corresponding to a measured force in the range of 16-20 N on thescoring wheel. The targeted vent depth for both sheets is in the rangeof 140-175 μm.

The scribing speeds employed are 250 mm/s for the sheet with the 30 μmcompression layer depth and 125 mm/s for the sheet with the 36 μmcompression layer depth. The first scribe direction is parallel to thelong dimension of each sheet and the second or crossing scribe directionis parallel to the short dimension of each sheet.

Following the machining of crossing vent lines in each of the sheets,each sheet is successfully separated into four sheet segments of smallersize by the application of a uniform breaking force across the ventlines in each sheet utilizing a mechanical breaker bar. A breakingstress in the range of 25-35 MPa is sufficient to achieve sheetseparation along each of the vent lines.

Sheets of the same glass composition, but of reduced (0.7 mm) thicknessand incorporating thinner surface compression layers, are alsosuccessfully cross-vented utilizing the same glass cutting equipment asdescribed above, but at scoring speeds in the range of 250-500 mm/s.Additionally, utilizing Gen-5 glass cutting equipment from the samemanufacturer, successful cross-scribing and sheet separation areaccomplished without edge-crushing the sheets to provide the requiredcrack initiation sites. In that procedure, suitable crack initiationsites are formed proximate to the edges of the larger sheets byutilizing slow abrasive wheel set-down velocities and light wheelscribing loads at wheel set-down locations greater than 5 mm from theedges of the larger glass sheets.

FIG. 4 of the drawings is an optical photomicrograph presenting anelevational side view of a section of a fracture surface formed upon theseparation a chemically strengthened glass sheet segment from a largersheet of chemically strengthened glass in accordance with the methodshereinabove set forth. The horizontal bar in the field of the fracturesurface represents a dimension of 500 μm. The vent line giving rise tothe stress fracture that produced the clean separation surface shown inFIG. 4 appears as a bordering band along the upper edge of that surface.

The foregoing descriptions and examples demonstrate that chemicallystrengthened glass sheet may be successfully segmented into smallerstrengthened sheets of any required size and shape, even using existingmechanical scribing systems, provided that procedures effective toovercome the glass breakage problems presented by the high sheet stresslevels and frangibility of chemically strengthened glass sheets areappropriately applied. Of course, those examples and descriptions aremerely illustrative of the range of scribing and separation proceduresthat may successfully be adapted for the purpose of processing largesheets of chemically strengthened glass within the scope of the appendedclaims.

1. A method for scribing a strengthened glass sheet comprising the stepsof: forming a crack initiation site in a first surface and proximate toa first edge of a glass sheet, the sheet comprising a surfacecompression layer of layer depth DOL and the crack initiation sitecomprising a surface indentation extending into the first surface; andmechanically scoring the first surface from the crack initiation site atoward a second edge of the glass sheet to scribe a vent line extendinginto the first surface to a vent depth at least equal to DOL but lessthan a fracture depth causing separation of the glass sheet.
 2. A methodin accordance with claim 1 wherein the surface indentation extends intothe first surface to a depth h that is greater than DOL but less than 3times DOL.
 3. A method in accordance with claim 1 wherein thestrengthened glass sheet has a thickness not exceeding 1.5 mm.
 4. Amethod in accordance with claim 3 wherein the surface compression layerexhibits a peak compressive stress in the range of 400-900 MPa, a peakcentral tension of less than 50 Mpa, and wherein layer depth DOL is inthe range of 5-100 μm.
 5. A method in accordance with claim 3 whereinthe peak central tension does not exceed 40 MPa and the vent depth isthe range of about 10-20% of the thickness.
 6. A method in accordancewith claim 3 wherein the surface indentation extends into the firstsurface to a depth that is less than 10% of the thickness.
 7. A methodin accordance with claim 1 wherein a plurality of vent lines extendingfrom a plurality of crack initiation sites are scribed in the firstsurface of the glass sheet.
 8. A method in accordance with claim 7wherein at least one vent line crosses at least one other vent line. 9.A method in accordance with claim 1 wherein the glass sheet has a widthW and a length L that is greater than the width W, and whereinmechanical scoring comprises scribing a plurality of spaced vent linesfrom a plurality of crack initiation sites along directions parallel tothe length L.
 10. A method in accordance with claim 9 wherein mechanicalscoring further comprises cross-scribing at least one vent line from acrack initiation site proximate to a second edge of the glass sheetacross the plurality of spaced vent lines in a direction parallel to thewidth W.
 11. A method in accordance with claim 1 wherein mechanicalscoring comprises scoring the first surface with a rotating abrasivescoring wheel.
 12. A method in accordance with claim 11 wherein thescoring wheel comprises a tapered circumferential cutting edge.
 13. Amethod in accordance with claim 12 wherein the circumferential cuttingedge has a taper angle α in the range of 90-140°.
 14. A method inaccordance with claim 13 wherein mechanical scoring comprises moving thescoring wheel with respect to the first surface at a scoring speed inthe range of 50-500 mm/s while applying a scribing load in the range of10N to 30N to the first surface.
 15. A method of separating astrengthened glass sheet incorporating surface compression layers intotwo or more sheet sections comprising the steps of: scribing a vent lineextending into the first surface to a vent depth at least equal to adepth DOL corresponding to the depth of a surface compression layer onthe first surface, but less than a fracture depth causing separation ofthe glass sheet; and applying a uniform bending moment across the ventline of a magnitude sufficient to separate the glass sheet along thevent line.
 16. A method in accordance with claim 15 wherein the step ofscribing a vent line in a first surface of the glass sheet comprises thesteps of forming a crack initiation site in a first surface andproximate to a first edge of the glass sheet, the sheet comprising asurface compression layer of layer depth DOL and the crack initiationsite comprising a surface indentation extending into the first surface;and mechanically scoring the first surface from the crack initiationsite a toward a second edge of the glass sheet to scribe a vent lineextending into the first surface to a vent depth at least equal to DOLbut less than a fracture depth causing separation of the glass sheet.17. A method in accordance with claim 15 wherein the bending moment isapplied by cantilever beam loading.
 18. A method in accordance withclaim 15 wherein the bending moment is applied by three-point bending.19. A method in accordance with claim 15 wherein the bending momentgenerates a tensional stress a in the range of about 10-35 MPa acrossthe vent line.
 20. A method in accordance with claim 19 wherein thetensional stress is generated in three-point bending through theapplication of a mechanical force to a second surface of the glass sheetopposite the first surface.
 21. A method in accordance with claim 20wherein the mechanical force is applied to the second surface by abreaking bar aligned with the vent line.
 22. A method in accordance withclaim 21 wherein the glass sheet is supported on a compliant supportsurface as the mechanical force is applied.
 23. A method in accordancewith claim 22 wherein the vent line is positioned over a groove in thecompliant support surface during application of the mechanical force.